1. Field of the Invention
This invention relates to a system for generating electricity. More particularly, this invention relates to a system comprising fuel cell electrical output units having a configuration that results in more efficient electricity generation at lower costs than known systems. Although applicable to different types of fuel cells, the disclosed system is particularly suitable for use with planar fuel cells, in particular, planar solid oxide fuel cells.
Generally, fuel cell electrical output units are comprised of a stacked multiplicity of individual fuel cell units separated by bi-polar electronically conductive separator plates. Individual fuel cell units are sandwiched together and secured into a single staged unit to achieve desired fuel cell energy output. Each individual cell generally includes an anode electrode and a cathode electrode, a common electrolyte, and fuel and oxidant gas sources. Both fuel and oxidant gases are introduced through manifolds, either internal or external to the fuel cell stack, to the respective reactant chambers between the separator plate and the electrolyte.
2. Description of Related Art
Solid oxide fuel cells have grown in recognition as a viable high temperature fuel cell technology. There is no liquid electrolyte with its attending metal corrosion and electrolyte management problems. Rather, the electrolyte of the cells is made primarily from solid ceramic materials so as to survive the high temperature environment. The operating temperature of greater than about 600° C. allows internal reforming to convert hydrocarbon fuels into hydrogen required for the reaction, promotes dell reactions with non-precious materials, and produces high quality by-product heat for cogeneration or for use in a bottoming cycle. The high temperature of the solid oxide fuel cell, however, places stringent requirements on its materials. Because of the high operating temperatures of conventional solid oxide fuel cells (approximately 1000° C.), the materials used in the cell components are limited by chemical stability in oxidizing and reducing environments, chemical stability of contacting materials, conductivity, and thermomechanical compatibility.
Planar solid oxide fuel cells have the potential to be more efficient and lower in cost than tubular designs because the cells used have shorter current paths and are simpler to manufacture. However, as suggested above, it is difficult to find suitable low-cost materials for the sealant and interconnect for use at the 1000° C. solid oxide fuel cell operating temperature. Thus, to enable the use of lower cost materials, it is desirable that the operating temperature of the solid oxide fuel cells be reduced.
An effective heat integration between fuel cell stack heat removal and air preheating has been a major challenge for the solid oxide fuel cell. Standard heat integration schemes employed by conventional systems use the cathode gas for the heat removal and preheat the air feed by heat exchange with the cathode exhaust gas. As the temperature rise of the cathode gas in the stacks is limited (usually less than 100° C.), the required cathode flow for the stack heat removal is very large. Typically, a stoichiometric air ratio of 4-5 is necessary to provide the cathode flow required for the heat removal. This large air flow significantly increases the air preheater size. The large size, in conjunction with the high air discharge temperature required, significantly increases the air preheater cost. This is one major reason for the high cost of solid oxide fuel cell systems. The large air flow also increases the system pressure drop. The combined effect of large flow and high pressure drop increases the air blower size and the auxiliary power consumption. Consequently, the efficiency of the system is reduced.